Often, integrated circuit (IC) designers and manufacturers measure voltages and currents deep inside an operating IC to analyze and repair operational faults. In particular, when a prototype IC is not performing up to specifications or an IC is returned from the field due to a failure, engineers test the faulty IC to determine the cause of the failure and take corrective measures to avoid the problem in subsequently produced ICs. Two classic probing techniques involved: accessing exposed conductors with external physical probes and probing with a focused electron beam. Both require access to the conductors in metallization layers of the IC. While early ICs had few metallization layers, eight metallization layers are not uncommon in contemporary designs. As a result, the classic techniques require invasive and destructive processes to provide access to the conductors that risk compromising the accuracy of the measurement.
To avoid these problems, an optical probing technique developed to non-invasively measure and debug electrical activity of an IC. The technique is generally known as picosecond imaging circuit analysis (PICA). PICA captures emissions of photons produced by carriers traversing through an electric field between high and low voltages. The intensity of the photon emissions is linearly related to the current. More specifically, the voltage difference between the ends of the channel, e.g., the drain and source, of a field effect transistor subjects carriers in the channel to an acceleration force that increases the kinetic energy of the carriers. Most carriers collide into other carriers, substantially reducing the carriers' kinetic energy upon reaching the other end of the channel. However, carriers that encounter very little impedance while traversing the channel may attain a kinetic energy that exceeds the bandgap energy for the channel, e.g., silicon. As a result, the carrier may release a photon, which can escape the IC.
Several current trends in IC design have reduced the foreseeable viability of PICA. Current IC designs often implement Complementary Metal Oxide Semiconductor (CMOS) circuitry to reduce power consumption by ICs. The tendency to move toward CMOS has reduced the currents in conventional ICs. In particular, CMOS circuits are designed to draw current only during switching activities. Thus, most photons are released while components of the CMOS circuits are switching from one state to another.
Further, designers continually try to reduce the size while increasing the speed of the ICs. The trend is to reduce the size of each component of the ICs, which also requires that the operable circuit voltage, Vdd, has reduced over generations of IC designs. The problem is that the reduction in Vdd reduces the average excess in kinetic energy over the bandgap energy, which, in turn, makes the photon emissions resulting from switching activities less distinguishable from normal thermal activities of the IC, often referred to as noise. The normal thermal activities include, for example, leakage currents.
Detectors designed to detect the photon emissions may include filters to exclude the noise. However, the relatively few numbers of photons released during an operation of the IC are currently inadequate for accurate analysis of circuit activities. Thus, multiple repetitions of the same circuit operations may be required to determine the problem with the faulty IC. It's not uncommon for a test to require 20 hours before sufficient data is recorded to analyze the problem.
PICA detectors are improving in sensitivity but have limited ability to detect high frequency signals. To probe states of high frequency signals with critical timing, like the clock, strong photon emissions are needed to be distinguishable with accuracy in captured imaged over time. As beneficial as PICA is to an integrated circuit debugging, the problem of low energy photon emissions make the benefits difficult to realize. Therefore, a new non-invasive analytic tool to measure the electrical activity of an integrated circuit having a low supply voltage is needed.